The next generation of X-ray imaging is seeing “X-ray colours” with spectrally sensitive detectors being used in acquiring images, as compared to the traditional “greyscale” imaging using spectrum-integrating detectors. The ability to get spectrally differently weighted information from the same image view enables well-known and broadly discussed new diagnostic features, for example, material separation, material recognition, beam hardening correction, and quantitative imaging.
For spectral imaging the exact knowledge of the exposing X-ray spectrum is required. Even very small spectral variations, for example in the ratio between high energy and low energy X-ray photons, will cause severe artifacts in diagnostic features, like material separation.
Typically, an exact tube spectrum is measured during a calibration procedure, however there are many effects which influence a spectrum on a very short time scale and which cannot be accounted for during an actual imaging process. For example, the applied tube voltage might drift during the imaging procedure, or voltage ripples of different strengths might occur.
US2014/0014828A1 describes a method and system for adaptive calibration for tomographic imaging systems.